Recently, containers made of PET resin represented by PET bottles and PET trays are used in a large quantity due to its excellent mechanical strength, transparency and high food safety and sanitation. However, the PET resin once molded into containers had been depolymerized by the heat upon molding to cut molecular chain. The lower molecular weight PET molecules thus produced degrades properties. Accordingly, recovered PET cannot be reused for containers for food again. Then, the recovered PET is recycled only to fiber materials, such as for cotton stuffing and cloths, egg cartons, building materials, or chemical recycles to a part of original chemical raw materials.
Some methods of improving properties of the lower molecular weight PET have been proposed. One method is to render high molecular weight by conducting solid phase polymerization of recovered PET flakes, which have been completely dried, at a temperature of 200° C. or more and a pressure of 7 kPa or less (Japanese Patent No. 4,013,031). Another method is to use a modifier which combines lower molecular chains to render high molecular weight (Japanese Patent Nos. 3,503,952, 4,255,838, JP 2006-176711 A). It is also known to conduct extrusion molding of PET resin without drying (JP 4-278310 A, JP 2004-155968 A).
However, the solid phase polymerization needs complete drying and a high temperature of 200° C. or more for a long time after the drying, which require a large amount of equipment cost and energy cost. The method of using a modifier has a problem of residual acetaldehyde, and is unfavorable in view of food safety and sanitation.